1. Field of the Invention
The invention relates to particulate packing material for use as dunnage for providing protection to articles during shipment by absorbing shocks and by isolation of the articles from the walls of the shipping containers by means of the packaging particles. The invention is based on the discovery that extruded foamed sheet of polymerizable vinyl aromatic monomers, such as styrene, having a skin on only one surface thereof can be caused to warp or distort to provide curved configurations by the application of heat at temperatures at and above the foaming temperature. The present invention is also based on the discovery that the direction of axial orientation of the sheet in respect to the longest or shortest dimension (not thickness) of the particle effects the shape and ultimate curved configuration of the warped or distorted packaging particle after heating. Thus, small strips of foamed thermoplastic sheet having a skin on one side only and having a major dimension in the machine direction, e.g., in the direction of extrusion, provides substantially different shapes and curved configurations than particles made by heating strips cut from foamed sheets transversely to the machine direction. Furthermore, it was discovered that packaging particles made by heating strips cut from such foamed sheets on a bias to the machine direction are significantly different in shape from either of the above-mentioned particles and that packaging particles made by heating plastic sheet pieces having no longest or shortest dimensions are also significantly different in shape than any of those mentioned above.
The present invention permits the production of packaging particles which are free-flowing, which do not tend to interlock and which are of exceedingly low bulk density.
The present invention is particularly adapted for use in those circumstances where the foamed sheet having a skin on only one side thereof is produced at one location and sent to the location or area where the packaging particles will be made and used. In this manner, considerable shipping space and expense as well as storage space and expense is conserved because the foamed sheet disposed in rolls does not take up nearly as much space as the shaped, curved particles. At the use location or area, when the need arises for packaging particles, the rolls can be cut into the desired flat shapes and then heated to the foaming temperature or above to cause further distortion or warping and foaming of the particles into the desired curved configuration. The foamed sheet can be cut into any desired patterns to produce various and sundry shapes and configurations. Additionally, adjustments to the biaxial orientation and the relation of the major and minor dimensions (not thickness) to the direction of orientation can result in an even grater variety of different forms and curved configurations.
The present invention is carried out by forming on a plastic sheet being extruded a skin of higher density material than the remaining portions of the sheet. The advantages and benefits of the invention are not obtained when no skin is formed on either surface of the sheet and are not obtained when a skin of substantially the same density is formed on both surfaces of the sheet. The advantages and benefits of the invention result only when the skin of high density material is formed on just one but not both surfaces of the foamed extruded sheet.
2. Description of the Prior Art
There exists a worldwide market for a product generally referred to as packaging particles or plastic dunnage. Other terms for the product are "loose fill" and "free flow". The first product used in this application was popcorn. Present day packaging particles are usually made of polystyrene foam. More specifically these particles are made of a very small celled foam with an actual density of approximately 1 pound per cubic foot. They are reduced to this density from their extruded, expanded form by passing the extruded, expanded form through a steam expander such as that which is used in the steam chest molding process of molding foamable polystyrene beads. The expander is a unit which subjects the particles to direct steam action which is very effective in reducing the density of polystyrene foams.
Many attempts have been made to provide packaging particles and particles made from plastics, such as foamed polystyrene, have been marketed. U.S. Pat. Nos. 3,723,240, 3,400,037 and 3,251,728 and the patents mentioned therein, illustrate the prior attempts to produce packaging particles some of which have been marketed and others of which have not. U.S. Pat. No. 3,723,240 describes several of these prior art attempts. This patent itself relates to the treatment of foamable strands of polystyrene to impart asymmetry to the strand prior to expansion by heating, that is, the strand is treated on one side thereof so that it expands on that side to a lesser degree. The treatment can be a scraping treatment, a heta-deforming treatment, the application of a plasticizer, and the positioning of a relatively non-foaming or relatively low foaming element eccentrically in the strand. During subsequent heat treatment, the elongate strand forms a foamed coil strand having a helical configuration. This patent, however, fails to disclose or suggest the utilization of foamed but still foamable sheets of thermoplastic resins, such as styrene and having only one surface thereof formed with a skin, or more dense layer, to produce a wide variety of different configurations. Furthermore, the production rate of the method of this patent is comparatively low whereas the packaging particles of the present invention can be produced at much greater rates of production.
U.S. Pat. No. 3,400,073 teaches a method for making packaging particles by extruding filaments, rods, or strands followed by slicing same and heating the sliced chips to expand same. The method of this patent is limited to low production rates as compared to production rates of the method of the present invention. In addition, the resulting packaging particles possess a relatively higher bulk density than some of the other prior art particles.
Other patents in the field of producing packaging particles include U.S. Pat. Nos. 3,026,273, 3,188,264, 3,281,895, 3,723,237. Additional patents in this general area are U.S. Pat. Nos. Re. 27,243, 3,066,382, 3,481,455, 3,829,269, 3,855,053, and 3,887,672.
Furthermore, the production of foamed sheets having a skin of unfoamed material, or of a higher density material than the remainder of the sheet thickness, has been known for many years as illustrated by U.S. Pat. Nos. 3,311,681 and 3,560,600, the disclosures of which are incorporated herein by reference. These patents produce a foamed plastic sheet by extruding an expanded tube or bubble of polystyrene foam sheet and contact one surface of the tube with cooling gas or cooling surface to congeal that surface with little or no foaming while the remainder of the tube thickness and the other surface continues to foam. There is no teaching or suggestion in these patents to cut the resulting foam sheet into sheet-like pieces and subject same to heat to post-expand them.
There has been much patent activity in the packaging particle field. Almost all of the patents have had to do with the shape of the particle inasmuch as the goal is to further reduce the bulk density of the particles to at least as low as 0.5 pounds per cubic foot. Bulk density is measured by placing the particles loosely in a cube container such as a 12 .times. 12 .times. 12" box. The weight of the particles for this cubic foot volume is the bulk density and includes the air voids around the particles. Therefore, a particle with a shape that allows for much void area would have substantial economic value since the product is measured and used on a volume basis rather than a weight basis. Hence a useable particle with the lowest bulk density is most desirable. Most of the patents mentioned above relate to extruded strands of a constant cross section. These cross-sections are circular (Alta-Pak), a muted "S" (Pelapan Pak), a trilobular (Pakon, Tektronix) and a double tube or an 8 shape (Foam-Pak). All of these are strands extruded continuously and passed through a cutter station so that the constant cross section strand is cut into 1/2 inch or so lengths. There are variations to this cutting length since some products are produced so that in the unfoamed state they may be 1/8 to 1/4 inch in length. Other products are sliced and curl during the post-expansion. It is these cut-off sections of foam particles that are post-expanded as mentioned earlier.
One of the more serious problems with the just mentioned prior art particles is production rate. Since the expansion is great, the actual orifices from which they are extruded are quite small. A single orifice will produce about 10 pounds per hour. A smaller extruder runs with seven such strands for a total production rate of 70 pounds per hour. This is typical of the size extruder and rates used in producing the prior art particles. A foam line with a production rate of 300- 600 pounds per hour would be almost unmanageable in terms of the number of strands needed for this increased production rate.
By the process of this invention, particles can be produced as fast as any foam line is able to run. In other words, if a 41/2- 6 inches dual extruder foam line is capable of producing 600-700 pounds per hour of foam, then it could produce 600-700 pounds per hour of packaging particles.
One of the other objections to the packaging particle business is the shipping and storage of the prior art particles. A 15 cu ft bag holds only 71/2 pounds of post-expanded particles. Therefore, shipping and storage of the prior art particles can be an immence undertaking. The process of this invention has the ability to overcome this problem too.